Rigid polyurethane (PU) foam is widely used in appliance and building industries due to its excellent thermal insulation property. Using rigid PU foam with improved thermal insulation performance is one objective for appliance manufacturers. It is known that the thermal conductivity (Lambda, λ) of rigid PU foam is attributed to at least heat conduction through the gas contained in the rigid PU foam (gas conductivity), conduction through the solid structure of the rigid PU foam (solid conductivity) and from the radiant heat transfer of the rigid PU foam. In conventional rigid PU foams used for appliance, gas conductivity accounts for about 60-70% of the total lambda value. One conventional method to minimize gas conductivity is to use certain types of blowing agents such as hydrochlorofluorocarbons (HCFC, e.g., HCFC141b), hydrofluorocarbons (e.g., HFC245fa), hydrofluoroolefines (HFOs), hydrocarbons (e.g., c-pentane), and mixtures thereof in the production of the rigid PU foams. Some of these gases, however, are known to have ozone depletion potential (ODP) or global warming potential (GWP).
Another approach to minimize gas conductivity is to limit the number of energy exchanging collisions between gas molecules in the cells of the rigid PU foam. Minimizing the number of collisions between gas molecules in the cells can effectively reduce gas conductivity without the use of HCFC, HFC, HFOs or hydrocarbons. To achieve this result the size of the cells of the rigid PU foam needs to be close to or smaller than the mean free path of gas molecules between collisions. This is known as the “Knudsen effect” and can be achieved either by reducing the size of the cells, by reducing the gas pressure inside the cells, or both.
Foaming methods used with rigid PU foams do not, however, readily achieve cell size below about 180 micrometer (μm). For such foams, strong vacuum needs to be applied (<1 mbar, often <0.1 mbar) to achieve conditions under which the Knudsen effect becomes significant. Thus, there is a need for rigid PU foams having small cells that can achieve low thermal conductivity values (e.g., less than 18 mW/m-K) without the need of very strong vacuum or for the use of gases that have ODP or GWP.
As a green blowing agent with zero ODP and negligible GWP, supercritical carbon dioxide (ScCO2 ) has drawn the interest in foaming industry and may be a promising blowing agent in producing microcellular or even nano-cellular foams for thermoplastic polymers. It has been suggested that ScCO2 can also be used in thermosetting polymers. For example, ScCO2 has been used in preparing rigid PU nano-foams. A surfactant is typically used in preparing rigid PU nano-foams with ScCO2. The loading level and type of surfactant used in preparing rigid PU nano-foams can have an impact on the quality of the resulting rigid PU foam. Fluorocarbon and/or fluoroether based surfactants are known to be compatible with PU foaming systems that use ScCO2. These perfuloro-based surfactants, however, can be very expensive and more concerning may escape from, the PU foams due to their low molecular weight. As such, the use of such perfuloro-based surfactants is limited. Thus, there is a need in the art to develop perfluloro-based surfactants that have a high performance-to-cost ratio.